Update docs for new gitlab-experiment gem location

The following discussion from !94129 (merged) should be addressed:

• @pcalder started a discussion: (+2 comments)

  @kwiebers I also opened https://gitlab.com/gitlab-data/analytics/-/merge_requests/5845

  cc'ing you as you were involved in when we first added it https://gitlab.com/gitlab-data/analytics/-/issues/7116, and to check if there are other changes you can think of that we might need to consider.